The present invention relates generally to using Digital Signal Processing (hereinafter "DSP") techniques to filter a modem signal and more particularly to recovering a modem signal that has been altered by channel distortion and enhancing the performance of the modem using DSP techniques.
Traditionally, telephone lines have been used to communicate voice signals. Telephone lines have provided a fairly clear and reliable means of communicating voice information. However, each telephone line has its electrical characteristics, and some of them can introduce severe distortion, such as attenuation and phase delay, etc., to the transmitted signal. In voice communication, such as telephone conversations, the human ear can compensate for the distortion to a certain degree.
Recently, telephone lines have been used for data communication (e.g., e-mail messages or Web pages). For data communication over the same telephone line, the modem can not tolerate the same level of distortion as the human ear. Usually, the demodulator in a modem can tolerate a certain level of distortion without making a bit error. The amount of channel distortion that can be tolerated by a modem is mainly determined by the modulation scheme, the error correction code it uses, or the way the signal is encoded.
Error rate is a term used in digital transmission systems to describe the amount of error present during a particular transmission. Voice and data communication can tolerate a certain amount of error or disruption in a transmission and still have the recipient receive an intelligible message. For example, voice communication can tolerate a bit error rate of 10.sup.-2 while data communication can tolerate a bit error rate of 10.sup.-7. Hence, the transmission in data communication needs to be more accurate than voice communication for a recipient to receive an intelligible message. Therefore, data communication needs to be accurately transmitted for the recipient to understand the message or to ensure that the recipient can retrieve the same message that was transmitted.
Frequency Shift Keying (hereinafter "FSK") modulation is a method used for transmitting data across a telephone line. Typically, when using FSK modulation, two different frequencies are transmitted such that when one frequency is transmitted for a certain period of time a logical "0" has been transmitted and when the other frequency is transmitted for a certain period of time a logical "1" has been transmitted. When channel noise or attenuation is present in a signal, different frequencies can be affected differently. Typically, the channel noise and attenuation need to be rectified for voice and data communication to be accurately received.